Download Earthquakes are concentrated along oceanic ridges, transform faults

9/16/14
•  Because Earth’s size has not changed, expansion
of the crust in one area requires destruction of the
crust elsewhere.
Earthquakes are concentrated along oceanic ridges,
transform faults, trenches and island arcs
•  Currently, the Pacific Ocean basin is shrinking as
other ocean basins expand.
•  Seismicity is the frequency, magnitude and
distribution of earthquakes.
•  Tectonism refers to the deformation of Earth’s
crust.
3-3 Global Plate Tectonics
•  Destruction of sea floor occurs in
subduction zones.
Figure 3-6a Global Distribution of Earthquakes
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•  Benioff Zone is an area of increasingly deeper seismic
activity.
•  It is inclined from the trench downward, toward the
deep-sea trench and away from the island arc.
•  Subduction is the process at a trench
whereby one part of the sea floor plunges
below another and down into the
asthenosphere.
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•  Earth’s surface is composed of a series of
lithospheric plates.
•  Plate edges extend through the lithosphere and
are defined by seismicity.
Figure 3-16b
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•  Plate edges are trenches, oceanic ridges and
transform faults.
•  Seismicity and volcanism are concentrated
along plate boundaries.
•  Movement of plates is caused by thermal
convection of the “plastic” rocks of the
asthenosphere that drag along the overlying
lithospheric plates.
Figure 3-7b The Lithosphere & the Asthenosphere
3-3 Global Plate Tectonics
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Figure 3-8 Plate Mechanics
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Mantle plumes originate deep within the
asthenosphere as molten rock which rises and
melts through the lithospheric plate forming a
large volcanic mass at a “hot spot.
Figure 3-9b Mantle Plume
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Wilson Cycle refers to the sequence of events
leading to the formation, expansion,
contracting and eventual elimination of ocean
basins.
•  Stages in basin history are:
–  Embryonic - rift valley forms as continent begins to split.
–  Juvenile - sea floor basalts begin forming as continental
fragments diverge.
–  Mature - broad ocean basin widens, trenches eventually
develop and subduction begins.
–  Declining - subduction eliminates much of sea floor and
oceanic ridge.
–  Terminal - last of the sea floor is eliminated and
continents collide forming a continental mountain chain.
Figure 3-9 Volcanic Chains and Mantle Plume
Figure 3-9c Kilauea, Hawaii
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The Wilson Cycle
Figure 3-10 Ocean-basin Evolution
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The Global Age Pattern of the Oceanic Crust
Figure B3-3 Age of the Ocean Crust
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•  The San Andreas fault in southern California is a
transform fault.
•  The Red Sea is a juvenile ocean basin that is forming as the
African plate diverges from the Arabian plate.
•  New basaltic ocean crust is just beginning to form in the center
of the Red Sea.
•  It connects the sea-floor spreading ridge of the Gulf of
California with the spreading ridge off Oregon and
Washington.
•  If these plate motions continue, Baja will splinter off
California.
•  Because the San Andreas fault has an irregular trace,
strike-slip motion can cause local compression or tension.
3-3 Global Plate Tectonics
Figure B3-3 Age of the Oceanic Crust
3-3 Global Plate Tectonics
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